1. Field of the Invention
The present invention relates to processes for the localization of a scintillation event in a gamma camera having a number of photomultipliers arranged over a camera surface, the output signals of which are compared with comparative signal sets which have been generated by comparative scintillation events with location-known origins thereby forming, a location-dependent probability function, with the location of the maximum of the probability function being registered as the location of the scintillation event.
2. Description of the Prior Art
Disintegration quanta of radioisotopes are traced in an examination object using a gamma camera. In such a gamma camera a scintillation crystal, usually consisting of sodium iodide (Nal(T1)) transforms the energy of the absorbed gamma quanta (typically 140 keV for .sup.99 Tc) into light. This scintillation light is distributed via a light guide, such as a glass plate of Pyrex.RTM., over a number of photomultipliers, the electric output signals of which are used for the determination of the absorption locus. In a conventional gamma or Anger camera a localization is undertaken according to the center of gravity principle. The absorption locus is obtained from the sum of the weighted individual signals of the photomultiplier divided by the sum of the signals of all the multipliers. The weighting factor is chosen dependent on the position of the corresponding photomultiplier relative to a coordinate origin on the camera surface. The division by the sum signal makes the localization as independent as possible from fluctuations in the light yield.
The localization according to the center-of-gravity principle is ordinarily realized with analog switching circuits, with the output signals of the photomultipliers being summed according to a weighing represented by resistances. Besides the resistive weighting processes are also known employing a capacitive weighting of the output signals. The sum signal, again, is formed by analog summing. The main disadvantage of this center-of-gravity localization is a correct determination of the absorption locus can be achieved only under certain conditions. For example, if in the border zones of the crystal the symmetry of the signal distribution is disturbed by light reflection on the crystal border, false localizations result. Moreover, for an absorption locus on the border the arrangement of the photomultipliers is weighted on one side toward the crystal interior, which leads to the result that the center-of-gravity localization in the border zone fails altogether. This is in part compensated in conventional gamma camera constructions, wherein the light guides and the photomultipliers overlap the crystal border. The consequence is that in the gamma camera with analog center-of-gravity localization there remains a border not usable for the image generation which, according to experience, has the extent of about 1.5 times the radius of a photomultiplier.
A process which, despite such border effects, permits a localization is described in an article by Milster et al.: "A Full Field Modular Gamma Camera" in the Journal of Nuclear Medicine, Vol. 31, No. 4, 1990, pp. 632-639. By digitization of the output signals of the photomultipliers directly after a pre-amplification, digital and nonlinear localization processes can be used. These processes make it possible to extend the localization directly to the crystal border and thus present possibilities for a modular construction of virtually arbitrary camera geometries.
In digital localization processes, the use of a probability function or a maximum likelihood estimator has proven useful; other processes such minimum average square error have been used with virtually equivalent results.
From the aforementioned Milster et al. article, it is known to employ a maximum likelihood estimator process on the basis of the Poisson model for statistical fluctuations of the output signals of the photomultipliers. For each possible combination of the output signals the most probable locus is entered in a memory in the form of a look-up table. Each position in the look-up table is additionally marked as to whether the value of the likelihood function falls into a certain likelihood window. This known realization of the maximum likelihood estimator process has the advantage that by means of the look-up table the localization can occur very speedily. The disadvantage is that because of the memory capacity requirement for the look-up table, the application is restricted to camera modules with few photomultipliers and a low bit-depth for the corresponding signals. For example, for a camera or measuring surface of 10.times.10 cm.sup.2 with four photomultipliers and a 5-bit representation of the output signals of the photomultipliers, a memory of 2 megabytes is required for the lock-up table. Only slight extensions of the above parameters lead to drastically higher storage requirements. Thus, for 4 photomultipliers with an 8-bit digitalization, 8 gigabytes are needed, while for 8 photomultipliers with a 5-bit digitialization, 2048 gigabytes are required. For still larger modules and/or higher accuracy of the quantization, the storage requirement increases into the immeasurable range.